Sell everything but your snipers: Using two 3:1 Snipers (first), 1 4:1 Sniper (strong), and one 0:0 bomb tower at the very beginning (first) is the way to do round 60. Works every time. Although you need to actually sell everything (even if you have cash to spare) as other towers actually get in the way.

thanks to tasslehof220 of the NK forums for this, it'll help you beat round 60!